Residence time or treatment time in biological unit cannot be overlooked!

Biological wastewater treatment relies on the underlying role of bacteria and other microbes as "nature's recycling system" as the microbes covert organics into ever smaller compounds and finally into carbon dioxide, water, and trace elements. It is this recycling role that we harness in waste treatment to rid wastewater of potentially toxic compounds that could impact downstream water health.

As discussed in earlier posts, the biological treatment unit provides an "ideal" environment to encourage microbial growth. The goal being to maintain a highly active, extremely concentrated population of microbes that can rapidly metabolize (treat) influent wastes. What can derail this treatment process?

• Changes in influent makeup (an example being adding in a new waste stream that is different from existing influent)
• Rapid increase in influent concentration
• Mechanical issues with equipment
• Hydraulic washout of biomass
• Temperature extremes in unit (either above 42oC or below 12oC seem to be problem points)

This brings me to the most frustrating (to me at least) source of problems in waste treatment. Simply, the lack treatment time in the biological unit. No matter how much you coddle the microbes - providing excess D.O., nutrients, increasing recycle rates, etc. Biological treatment of organics, especially recalcitrant, toxic, quasi-toxic or xenobiotic compounds, takes prolonged exposure to microbes for the multiple steps in biological transformation to occur.

So if you have a high COD wastewater with only a 4 hour residence time in the biological unit, you will get a reducing in highly soluble, easily degradable compounds. The more complex and less bioavailable compounds simply pass through or are incompletely metabolized. Sometimes the metabolites from initial degradation can be more toxic than the initial compound.

How much time is needed? This is where you do lab based treatability testing before designing a waste treatment unit. If you must build a unit with 2 - 4 hour residence time for industrial wastes, (Yes I often see this) - do not expect real biological treatment no matter what you do. You can "contact stabilize" where microbes adsorb some organics for later biodegradation in a digester. But most likely, you will have a simple physical treatment unit that aerates/mixes the influent only.